1. Field of the Invention
The present invention relates to a method of manufacturing a shadow mask used in a color picture tube and, more particularly, a method of manufacturing a shadow mask in which apertures for transmitting electron beams are formed by two-step etching.
2. Description of the Related Art
Shadow masks commonly used for color cathode ray tubes have a large number of apertures. These shadow masks are used to allow three electron beams each corresponding to red, green and blue emitted from the electron gun to impinge on each corresponding phosphor through the apertures.
A shadow mask according to the present invention is conventionally arranged in the following color picture tube.
The shadow-mask type color picture tube, as shown in FIG. 1, has an envelope constituted by a panel 1 and a funnel 2 integrally connected to the panel 1. A phosphor screen 3 constituted by tricolor phosphor layers is formed on the inner surface of the panel 1. A shadow mask 4 is arranged to be opposite to the phosphor screen 3 inside the panel 1.
The shadow mask 4 is formed by pressing in a predetermined various of curvature and a mask frame 10 welded to the peripheral portion of the shadow mask 4. The shadow mask 4 has a structure which selects three electron beams 7B, 7G, and 7R radiated from an electron gun 6 arranged in a neck 5 of the funnel 2 and allows these beams to be impinge on the tricolor phosphor layers. For this reason, a large number of apertures for transmitting electron beams are formed in the shadow mask 4 to have a predetermined arrangement. In addition, each of apertures 11 of the shadow mask 4 is formed such that a large opening 12 formed in one surface opposite to the phosphor screen communicates with a small opening 13 formed in the other surface as shown in FIG. 2 to decrease the amount of electron beams which collide with the inner walls of the apertures and are reflect in the undesired direction of the phosphor screen.
The shadow mask 4 is manufactured as follows. For example, apertures are perforated by photoetching in a shadow mask substrate such as a thin low-carbon steel plate having a thickness of, e.g., about 0.13 mm, a flat mask is made and this flat mask is formed by pressing after annealing.
Apertures are perforated in the shadow mask 4 as follows. First photoresist films are formed on both the surfaces of the shadow mask substrate. A pair of negative printing plates on which patterns of apertures each having predetermined shape and size are brought into contact with the photoresist films on both the surfaces of the shadow mask substrate, respectively, and the photoresist films are exposed. Thereafter, the patterns of the negative printing plates are printed and developed, thereby forming resist films having patterns corresponding to the patterns of the negative printing plates. The aperture patterns correspond to the large opening 12 and the small opening 13 which have the sections shown in FIG. 2, respectively. Thereafter, both the surfaces of the shadow mask substrate on which the large opening resist pattern layer and the small opening resist pattern layer are formed are simultaneously etched, thereby perforating apertures.
In recent years, a high-definition color picture tube has been developed. In accordance with this, a shadow mask having a small aperture diameter has been demanded. In contrast to a method of manufacturing a conventional shadow mask, a method called two-step etching disclosed in Japanese Patent Application No. 58-176378 is known as a method used for manufacturing a shadow mask having a small aperture diameter and used in the high-definition color picture tube. According to this method, as in the conventional method of manufacturing a shadow mask, photoresist films are formed on both the surfaces of a shadow mask substrate, negative printing plates are brought into contact with the photoresist films, respectively, and the photoresist photosensitive films are exposed and developed, thereby forming etching masks having patterns corresponding to the patterns of the negative printing plates. Thereafter, as the first etching, a protection film is adhered to one surface of the shadow mask substrate on which a large-opening resist pattern is formed. The other surface on which a small-opening resist pattern layer is formed is etched to form recesses of small openings. Thereafter, the small-opening resist pattern layer is removed, and an etching resistive agent containing an epoxy resin, casein, and a leveling agent as main components is coated on the small opening-side, thereby forming an etching resistive layer. In addition, the protection film adhered to the large-opening resist pattern layer is peeled. Thereafter, as the second etching, the large-opening side is etched to cause the large openings to communicate with the small openings, thereby forming apertures.
In a conventional single-step etching, an etching solution flows into an aperture rapidly as soon as the aperture is dug through by etching and passes through an aperture freely after communicating the large opening with the small openings, thereby undesired large aperture may be formed.
On the other hand, when the above two-step etching is used, since the etching resistive layer also prevents an etching solution to pass through an aperture, highly precise apertures can be obtained even when each of the apertures is designed to have a diameter smaller than that of the aperture obtained by conventional etching.
As described above, in the two-step etching, when the large-opening side is etched, the small openings side must be protected from etching. More specifically, as an etching resistive layer for protecting the small opening side during the second etching, an etching resistive agent containing an epoxy resin, casein, and a leveling agent as main components is often used. However, this etching resistive agent is not easily removed after the apertures are formed, and the formed apertures clog with the etching resistive agent.
A caustic solution is generally used to remove the etching resistive layer. However, since the etching resistive layer does not dissolve in the caustic solution, it is not easily removed. The removed etching resistive layer is suspended in the solution and attached to the shadow mask substrate, thereby easily clogging the formed apertures in the substrate.